1. Field of the Invention
The present invention relates to a power transmission apparatus for four-wheel drive vehicle for absorbing a rotational speed difference between the front wheels and the rear wheels or the left side wheel and the right side wheel by means of oil pressure generated in a hydraulic pump.
2. Description of Related Art
A four-wheel drive vehicle running by transmitting a driving force of an engine to all four wheels is of course superior in traveling stability on a road lower in a coefficient of friction, such as a snow-covered road or a frozen road, and a bad road, such as a gravel road or a dirt road, and also, has high traveling stability in the acceleration and deceleration and the high speed running even when traveling on a paved road. Such four-wheel drive vehicle has hitherto been especially highlighted as the vehicle realizable of comfortable traveling in spite of any road condition, natural condition, such as weather condition, and running state.
The four-wheel drive vehicle is basically realizable by directly connecting four wheels of front, rear, left side and right side to an engine of driving source respectively. In this case, different turning paths between the front and the rear wheels or between the left side and the right side wheels make it impossible to absorb a rotation speed difference generated between both wheels during the turning. Especially, during the sharp turning, the rear wheel inside the turning circle causes a slip and is dragged: so-called tight corner braking phenomenon occurs, thereby creating the problem that the turning performance deteriorates. Therefore, recently, such four-wheel drive vehicle has been dominant that is provided with a power transmission apparatus which can absorb the rotation speed difference between the front and the rear wheels or the left side and the right side wheels and distribute the driving force to both wheels corresponding to the rotation speed difference. A hydraulic pump for generating oil pressure corresponding to the rotational speed difference is used as one of the power transmission apparatuses, which is so constructed that a rotor connected in association with one of the front and the rear wheels is housed in a casing connected in association with the other, thereby causing between the rotor and the casing relative rotation corresponding to the rotational speed difference between both wheels. Magnitude of oil pressure generated within the hydraulic pump, with respect to the characteristic thereof, corresponds to the speed of relative rotation between the rotor and the casing, in other words, the extent of rotational speed difference between the front and the rear wheels or the left side and the right side wheels. Since the generated oil pressure acts between the rotor and the casing to restrict the relative rotation, the driving force corresponding to the rotational speed difference between both wheels is transmitted from one of the front and the rear wheels or the left side and the right side wheels to the other, thereby the four-wheel drive state is realized.
In such four-wheel drive device, in order to prevent generation of the aforesaid tight corner braking phenomenon, it is earnestly desired to reversibly obtain low generated oil pressure so as to loosely connect both wheels when the rotational speed difference between the front and the rear wheels or between the left side and the right side wheels is smaller. When the rotational speed difference between those wheels is larger, it is earnestly desired to reversibly obtain high generated oil pressure so as to tightly connect both wheels. In order to meet such requirement, adjusting means for adjusting oil pressure generated in the hydraulic pump must be used. Herein, since both the rotor and the casing rotate, when adjusting means for adjusting oil pressure is provided either at the rotor or the casing, it is difficult to perform the adjustment.